Microwave ovens, now a more or less permanent fixture in many home kitchens, increasingly find use in high volume industrial applications. For example, the tempering of large quantities of frozen meat, fish, poultry and fruit is greatly enhanced with the use of microwave ovens. Not only do microwave ovens provide for greater predictability in processing, they also eliminate an otherwise several hour wait time to thaw a frozen product prior to its availability for use, while minimizing drip loss and improving sanitation.
It has been known for some time that microwave ovens preferably include some type of structure for promoting uniformity of microwave energy distribution within the cooking cavity. This is because in a typical box shaped microwave oven, the spatial distribution of the microwave energy tends to be non-uniform. As a result, hot spots and cold spots are produced at different locations. Cooking results are therefore unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. This problem becomes more severe with foods which do not readily conduct heat from the areas which are heated by microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory results if the distribution of energy within the cavity is not uniform.
The conventionally accepted explanation for non-uniform cooking patterns is that electromagnetic standing wave patterns, known as modes, are set up within the cooking cavity. Within such a standing wave pattern, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent upon at least the frequency of the microwave energy used to excite the cavity and upon the dimensions of the cavity itself.
There have been a great many approaches proposed for alleviating the problem of non-uniform energy distribution. A common approach, used in many domestic microwave ovens, is a device known as a mode stirrer. A mode stirrer typically resembles a fan having metal blades. The mode stirrer rotates continually to alter the mode pattern within the cooking cavity. The mode stirrer may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material that is transparent to microwaves), or to conserve space within the cooking cavity, may be mounted within a recess formed in one of the walls adjacent the cavity. Another approach is to use a carousel tray within the oven cavity, which rotates the food itself.
Yet another approach to the problem of non-uniform energy distribution is to introduce a polarized energy beam into an oven cavity using a number of phased feed points, or to use an antenna including one or more planar conductive plates that are mechanically rotated.
Unfortunately, while these approaches work somewhat for power levels typical of microwave ovens intended for use in domestic kitchens, they are not particularly adaptable for use in high volume industrial applications. It is not uncommon for an industrial microwave oven, for example, to be required to process several hundred kilograms of product in a several minute time span, producing radio frequency energy levels of 50 kiloWatts (kW) or more. The known approaches have limitations in power handling due to the use of coaxial sections which intrinsically have less power capability than a waveguide. In addition, the rotating parts used to vary the energy polarization also have power handling and reliability limitations.
One solution to this problem has been described in U.S. Pat. No. 6,034,362 issued to Alton and assigned to The Ferrite Company, Inc. In that design, microwave energy is coupled to many modes of a microwave cavity by generating a circularly polarized microwave signal, whereby a polarization vector of the microwave energy continually rotates. The coupling device includes a transformer to match from an input waveguide polarization, such as provided by a rectangular waveguide, to a circular or square polarization waveguide section. The polarization waveguide section contains an asymmetrical insert element disposed within it such that in the region of the asymmetrical element, electromagnetic symmetry is about a symmetry plane only. The position and dimensions of the polarization insert are selected to introduce a difference in electrical phase of 90.degree. for polarizations which are parallel to and perpendicular to the symmetry plane.
By introducing two linearly polarized components which are 90.degree. out of phase with one another, the resulting sum of microwave energies produced at the output end of the polarizer is circularly polarized with constant amplitude, but with an angle of polarization that continuously rotates. Since the polarization vector continually rotates, microwave energy is coupled to many modes of the enclosure as a result.